Series capacitor protective equipment can employ a non-linear zinc oxide varistor to limit the magnitude of the voltage across the protected series capacitor. Under normal operating conditions load currents flow through the series capacitor such that the voltage across the capacitor is the product of the load current and the capacitive reactance. The voltage withstand of the capacitor is selected such that the capacitor voltage caused by the flow of load current is well within the voltage withstand capability of the capacitor. The varistor characteristic is selected such that under normal load current conditions the varistor current is limited to a few milliamperes. When a fault condition, for example a line to ground fault, occurs on the transmission line in which the series capacitor is connected to current through the capacitor increases. The current increase causes the capacitor voltage to increase and if the capacitor voltage is sufficiently high its voltage withstand capability is exceeded. To prevent the occurrence of excess voltage across the capacitor the zinc oxide varistor provides an alternative path for the fault current causing the excess capacitor voltage. However the current flow through the zinc oxide varistor during line fault conditions may cause damage to the varistor if allowed to continue for prolonged periods of time. Because excessive energy is dissipated in the varistors some means must be provided therefore for limiting the total energy dissipation within the varistor itself.
One means commonly employed to protect equipment from excess energy dissipation is the employment of a parallel air gap to bypass at least a part of the energy developed during a fault situation. One of the problems involved with the employment of triggered air gap devices is that a means must be provided to determine when the energy dissipated by the equipment becomes excessive. Another problem involved is to determine when the rate at which the energy is dissipated within the equipment becomes excessive. When the rate at which energy is dissipated in the equipment is too high the gap will not have sufficient time to operate before the equipment fails.
One of the purposes of this invention is to determine when the magnitude of rate of rise of energy dissipation is excessive and to provide low voltage pulses to initiate operation of a high voltage pulse generator for triggering an air gap when either of these conditions exist. A second purpose of the invention is to provide the low voltage initiating pulses at times when the voltage across the air gap is at or near its maximum value.